Explicit full configuration interaction (FCI) computations
in a double-zeta plus polarization (DZP) basis, involving
over 100 million Slater determinants, have been performed
to sample the potential energy curves of X
1Sigmag+ C2 and X
1Sigma+ CN+ for benchmark
purposes. Quartic force fields have been determined at the
optimized structures, and sets of anharmonic spectroscopic
constants [re, omegae, Be,
De, alphae, and
omegaexe] have been ascertained.
Analogous results obtained from high-level but inexact
correlation treatments establish a CISDTQ < CCSDT <
CISDTQPH ~ FCI series of increasing accuracy for the
notorious X 1Sigmag C2 and
X 1Sigma+ CN+
multireference systems. The data also reveal that recent
schemes for CI+PT extrapolations to the FCI limit are quite
accurate, to within 0.4 mH, 0.001 Angstroms, and 4
cm-1 in the total energy, re, and
omegae, respectively. Whether such schemes
approximate FCI curves with sufficient smoothness to
reproduce the anharmonic data obtained here is elevated as
a challenge for future work.